They're all different lengths. Pro tip I just learned: window counting is a better measure when looking at a listing than hoping they'll tell you the length. You'll see occasional posts here about "14-window" models, etc.
You can use the floor as-is but almost nobody recommends that. It's the source of much of a lot of the smell, and it's basically uninsulated. Many pull it up to look for and fix rust issues (but not all).
Wood stoves are great heating options. I'm going with sort of an insane plan with a bunch of heat sources and in-floor radiant, but if I wasn't, I would TOTALLY do a wood stove. My personal favorites are the mini Jotul 118 and the GrayStove.com one. Poke around - somebody else here just bought that one and has commented on it.
You're going to find there are lots of opinions here and no "right" answers. It's a very personal path and all the choices are yours. Although plenty of people will tell you you're wrong anyway.
Welcome! Where are you located?